The invention relates to a method of working the funnel portion of an envelope for a cathode ray tube, said funnel portion consisting of a conical portion and a neck portion. The funnel portion is supported and positioned at the level of two parallel, axially-spaced centering planes, so that the connecting line between the centers of the two centering planes forms the center line of the funnel portion. The funnel portion is blocked against rotation and subsequently a sealing edge is formed by grinding of the free edge of the funnel portion. The procedure is such that the plane of the sealing edge is at right angles to the center line of the funnel portion.
A method of this kind is known from U.S. Pat. No. 3,456,398. According to this known method, centering is realized at two centering planes by contacting the inner circumference of the funnel portion. Due to variations of the wall thickness and the interior dimensions, the centering planes are not accurately determined, which causes poor reproductibility and poor dimensional accuracy of the worked funnel portion, notably in the axial direction. Moreover, the inner circumference of the funnel portion is not available for use in positioning the tube during a later treatment such as the sealing of a face plate to the funnel portion. Because both centering planes are situated on the conical portion, the neck portion is not taken into account. The centers of the conical portion in the two centering planes determine only the center line of the conical portion and not the center line of the entire funnel portion, i.e. the combined conical and neck portions.